As shown in FIG. 1, the encode device in U.S. Pat. No. 6,100,519 comprises a light emitter 1, an encode wheel 2 and a light detector 3. Moreover, a ring-shaped lens 24 is arranged on the incident plane of the encode wheel 2 such that the light from the light emitter 1 and impinging on the incident plane of the encode wheel 2 can be focused by the ring-shaped lens 24. A plurality of cylindrical lens 25 with focusing direction vertical to that of the ring-shaped lens 24 are arranged on the exiting plane of the encode wheel 2 such that the light leaving the exiting plane of the encode wheel 2 can be further focused. Therefore, the working beam projected toward the chip 31 of the light detector 3 has less interference. Moreover, the light detector 3 can be designed to place between the focus point and the lens to prevent the fabrication difficulty and resolution reduction.
However, the encode device in above-mentioned application still has some limitations. For examples, the incident plane and the exiting plane for the working beam should be aligned with the light emitter 1 and the light detector 3. The encode device in above-mentioned application will be not applicable when the light emitter 1 or the light detector 3 is placed on one side of the rim of the encode wheel 2.
It is an object of the present invention to provide an improvement over the encode device in U.S. Pat. No. 6,100,519. A hollow dent is formed on the center part of the incident plane of the encode wheel. The inner circumference of the dent is provided with a ring-shaped lens such that the light impinging on the inner circumference of the dent is focused by the ring-shaped lens. The rim of the encode wheel is provided with a plurality of cylindrical lens which are perpendicular to the ring-shaped lens such that the light leaving the encode wheel can be further focused. Therefore, the working beam projected toward the chip of the light detector has less interference.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawing, in which: